1. Field of the Invention
The present invention relates to fiber optic external cavity strain sensors, including Fabry-Perot interferometric and intensity-based sensors and an intensity-based fiber optic sensor system using single mode fiber and bandpass filter as fiber optic sensor means and a method of measuring strain.
2. Description of Prior Art
In the past decade, fiber optic external cavity sensors (FOECS) have been developed for strain and temperature measurements. A FOECS comprises an input fiber and a wire bonded together in spaced axial alignment by a connecting sleeve wherein respective ends of the fiber and wire face one another and are spaced apart a predetermined distance to define an air gap. (A wire is a general term which refers to a wire shaped object of any material, such as a multimode fiber, a glass fiber, or a metal wire, etc.) The ends of the input fiber and the wire form two reflective surfaces. Physical conditions such as strain or temperature cause changes in the air gap, which modifies the reflected signal from the sensor. For strain measurement, a FOECS is bonded on a host structure whose deformation is to be measured. One major problem of strain measurement is temperature compensation. When temperature changes, the strain sensor will measure an apparent strain caused by the thermal expansion of the host structure. It is very important that a strain sensor can compensate the apparent strain and distinguish it from the mechanical strain of interest.
FOECS can be divided into two types, interferometric and intensity-based sensors. Known in the art are the following documents: MURPHY et al., "Quadrature phase-shifted, extrinsic Fabry-Perot optical fiber sensors", Optics Letters, Vol. 16, No. 4, p273, (1991); WANG and MURPHY, "Optical-fiber temperature sensor based on differential spectral reflectivity", Smart Mater. Struct. 1, p5, (1992). These documents relate to, respectively, using extrinsic Fabry-Perot to measure strain and intensity-based FOECS to measure temperature. There was no temperature compensation in the strain sensor.
Also known in the art documents: U.S. Pat. No. 5,202,939, Apr. 13, 1993, Belleville et al, "White-light interferometric multimode fiber-optic strain sensor"; Optics Letters, Vol. 18, No. 1, p78, (1993). These documents relate to using interferometric FOECS for strain measurement. The temperature compensation method applied in the sensor is only limited to a wire: a) which is made of the same material of the host on which the sensor is to be bonded, b) whose length covers the entire gauge length of the sensor. This method requires changing the material of the wire every time a different host is used. Furthermore, not all of the material that the host is made of can be pulled into a wire and polished to form an optical reflective surface at the tip.
One of the temperature compensation methods provided by the invention involves using a temperature sensitive reflector. Temperature sensitive reflectors have been used for temperature measurement before, rather than as an active temperature compensation method for strain measurement. A. Wang and K. A. Murphy, Smart Mater. Struct. 1, p5, (1992) and G. Boreman, R. Walters and D. Lester, SPIE, Vol. 566, p312 (1985) describe temperature sensors using interference filters whose transmission responses are functions of temperature.
Many types of systems have been developed for fiber optic sensors. Because of the periodical nature of the interference fringes, interferometric sensors require relatively complicated signal processing techniques in order to achieve absolute strain measurement, see, for example, C. Belleville and G. Duplain, Optical Letters, No. 18, p78, (1993). On the other hand, an intensity-based sensor measures the return light intensity changes from the sensor. An intensity-based sensor system must be able to distinguish the loss caused by the transmission line or connectors, etc. In the past, some intensity based sensors used a dual-wavelength measurement technique, which performs measurement in one wavelength and calibrates the system loss with the other wavelength, see, for example, E. Snitzer, W .W. Morey, and W. H. Glenn, Optical Fibre Sensors, Conf. IEE London, Pub. No. 221, p79 (1983); R. Jones and K. W. Jones, Opt. Eng., 27, p23 (1988); and A. Wang and K. A. Murphy, Smart Mater. Struct. 1, p5, (1992) . Such referencing method is vulnerable to differential transmission loss in wavelength caused by micro bend, especially for multimode fiber transmission lines, as discussed by Jones et al, system requires a reliable self-referencing technique.
A feature of the present invention is to provide practical temperature compensation methods for FOECS in strain measurement. These methods can be applied to all types of FOECS, interferometric or intensity-based. These methods allow temperature compensation for hosts of various materials, including concrete, composite materials, and metals, etc.
A further feature of the present invention is to provide a simple, reliable, and low-cost intensity-based FOECS and system for strain measurement. The FOECS and system may also be used for sensing other physical parameters that can cause the air gap changes in the sensor.